1. Field of the Invention
The present invention relates to an apparatus and to a process for delivering an abrasive suspension for the mechanical polishing of substrates.
2. Description of the Related Prior Art
The use of a silica suspension or alumina suspension for the polishing of silicon wafers on the rear face as well as the front face tend to be commonplace in the semiconductor industry, the optoelectronics industry and the optics industry. The problem naturally arises of how to deliver this product, from the supply drum, to several points of use. Such suspensions may also be used for polishing a metal layer already deposited on the silicon wafer; in this case, these suspensions may have physical or chemical properties that differ from the first.
The physico-chemical properties, viscosity, solids content and pH contribute to make these suspensions readily solidifiable if they are not periodically stirred. As a result, the pipes for delivering these suspensions often block up due to the latter solidifying, and the polishing of the silicon wafers exhibits defects characterized by deep scratches due to the detachment of agglomerates of solidified silica or alumina; the delivery pipes must therefore be frequently replaced and all this results in high operating costs and very low equipment utilization levels.
The various methods of delivering abrasive suspensions (or slurries) in the process of chemical and/or mechanical polishing of silicon wafers may be summarized as follows:
The most conventional method of delivering these abrasive suspensions consists in using pumps connected to pipes which take the product to the point of use. However, the nature of abrasive suspensions, especially because they contain abrasive particles on the one hand and because they either have a very acid or a very basic pH on the other hand, results in very rapid degradation of the pumps used, causing very serious maintenance problems: these delivery systems can only operate for a short period, most of the time being spent replacing, unblocking and maintaining the circuit. Even by using pumps based on polytetrafluoroethylene, the diaphragms in these pumps must be replaced every two to three months.
A system for the vacuum delivery of chemicals is known from U.S. Pat. Nos. 5,148,945 and 5,330,072, in which, instead of using pumps, intermediate containers are used which are put under vacuum sequentially so as to suck out the chemicals and to pressurize them in order to force them to be directed to the point of use. Although such a system does not use a pump, it is impossible with such a system to deliver abrasive suspensions such as those used in particular for removing metals on surfaces. This is because such suspensions have an extremely short lifetime before they solidify and because they are very corrosive.
An apparatus is also described, in Application WO 96/02319, which firstly comprises a measurement container into which the various chemicals used in the abrasive suspension are alternately taken, the said chemicals being quantitatively determined in this measurement container, each chemical, after being measured, then being sent to a mixing container in which all the compounds of the suspension are mixed in situ, this mixing container itself being connected to a container in which a negative pressure or a vacuum is created so as to suck out the suspension which is in the mixing container and send it to the various stations for delivering the abrasive suspension.
At the present time, there is a need for apparatuses distributing abrasive suspensions which do not require a great deal of maintenance and which may be available when the need arises.